Steel sheet for procelain enameling and method for production thereof, and enameled product and method for production thereof

ABSTRACT

Steel sheet for porcelain enameling having excellent workability yet capable of providing an enamel layer having excellent adhesion with the steel sheet on direct-on enameling once and still free of black specks defects, a method for producing the same, as well as a porcelain enamel product and the method for producing the same are provided, in which low carbon Al-killed steel sheet, high oxygen steel sheet, Ti-added steel sheet, Nb-added steel sheet, Ti—Nb-added steel sheet or B-added steel sheet is used. A steel sheet for porcelain enameling is produced by applying Ni—Mo alloy plating to the low carbon Al-killed steel sheet, high oxygen steel sheet, Ti-added steel sheet, Nb-added steel sheet, Ti—Nb-added steel sheet or B-added steel sheet having specified components and composition ratio, and after performing heat treatment thereto to control the content of Ni, Mo, and Fe that are present on the surface of the steel sheet in a predetermined range, porcelain enamel is applied and fired.

TECHNICAL FIELD

[0001] The present invention relates to steel sheet for porcelainenameling, method for producing the same, porcelain enamel products anda method for producing the same. In further detail, the inventionrelates to steel sheet having excellent adhesion with enamel and amethod for producing the same, as well as to an enamel product and amethod for producing the same, which uses low carbon Al-killed steelsheet, high-oxygen steel sheet, Ti-added steel sheet, Nb-added steelsheet, Ti—Nb-added steel sheet or B-added steel sheet.

BACKGROUND ART

[0002] Porcelain enamel products are widely used as kitchen andtable-top articles, components of heating appearance and components ofcooking appliance combustion equipment, bathtubs, interior and exteriormaterials of buildings, and the like. Porcelain enamel products aregenerally produced by firing twice; a ground coat enamel is firstprovided on the steel sheet and fired, a cover coat enamel is furtherapplied thereon, and fired again. In order to reduce the productioncost, however, a production method of direct enameling for only once(“direct-on enameling”) comprising providing the cover coat enameldirectly on the steel sheet followed by firing is employed. However, inthe production method of direct-on enameling, it is necessary to performpretreatment on the steel sheet before enameling, such as intensepickling and Ni dipping treatment. Further, as a steel sheet forobtaining favorable adhesion of the enamel products with the steel sheetin a direct-on enameling, a rolled sheet of high oxygen steel containingoxygen at a high concentration, which is produced by reducing C contentin the steel-making stage and produced by continuous casting withoutperforming deoxidation treatment, is widely utilized. However, a highoxygen steel generally suffers poor workability, and its application tousages requiring severe processing is limited.

[0003] As a means for improving the adhesion with the enamel in case ofapplying once, in Japanese Patent Publication No. 24413/1979 is proposedto perform, instead of the surface treatment of the conventionallyemployed nickel dipping method that is generally performed before enamelapplying, covering the surface of the steel material made of theconventional carbon steel sheet with an alloy of one or two types ofmetals selected from Ni and Fe with one or two types of metals selectedfrom Mo and W. However, there is no description on its workability.

[0004] On the other hand, in usages requiring severe workability such askitchen wares or bathtubs, rolled sheets of Ti-added steel or B-addedsteel have been used. However, as is disclosed in Japanese PatentLaid-Open No. 140286/1998, although Ti-added steel or B-added steel issuperior in workability, black specks defects generate in the enamellayer in case direct-on enameling is performed. Accordingly, the enamellayer had to be formed by means of ground coat enamel finishing or byapplying the cover coat and ground coat enameling in two times.

[0005] As described above, there is required a steel sheet for porcelainenameling available by reduced production steps and energy consumption,thereby reduced in production cost, yet improved in workability andhaving an enamel layer with high adhesion even in case the cover coatenamel is applied only once.

[0006] The present invention has been made in the light of thecircumstances described above, and the objects thereof are to providesteel sheet for porcelain enameling having excellent workability yetcapable of providing an enamel layer having excellent adhesion with thesteel sheet by applying direct-on enameling of cover coat enamel onlyonce and still free of black specks defects, to provide the method forproducing the same, as well as to provide a porcelain enamel product andthe method for producing the same, in which low carbon Al-killed steelsheet, high oxygen steel sheet, Ti-added steel sheet, Nb-added steelsheet, Ti—Nb-added steel sheet or B-added steel sheet is used.

DISCLOSURE OF THE INVENTION

[0007] In due course of the studies for solving the problems above, thepresent inventors have found that, by newly developing the compositionratio of the enamel layer and the surface treatment method, an enamellayer free of black specks defects and having excellent workability andadhesion with the steel sheet can be obtained by the direct-on enamelingeven on a Ti-added steel or B-added steel, which conventionally requiredfiring twice on forming the enamel layer. The present invention has beenaccomplished based on these findings.

[0008] More specifically, according to the first claim of the presentinvention for solving the problems above, there is provided a steelsheet for porcelain enameling characterized by that it comprises an Alkilled steel sheet containing 0.10% by weight or less (wherein, %represents “% by weight” hereinafter) of C, 1.0% or less of Mn, 0.15% orless of P, 0.1% or less of S, 0.1% or less of Al, and balance Feaccompanied by unavoidable impurities, having provided thereon a Ni—Moalloy plating film, and which is then subjected to heat treatment.

[0009] The steel sheet for porcelain enameling according to the secondclaim is characterized by that it comprises a high oxygen steel sheetcontaining 0.10% by weight or less of C, 1.0% or less of Mn, 0.15% orless of P, 0.1% or less of S, 0.1% or less of Al, 0.001 to 0.10% of O,and balance Fe accompanied by unavoidable impurities, having providedthereon a Ni—Mo alloy plating film, and which is then subjected to heattreatment.

[0010] The steel sheet for porcelain enameling according to the thirdclaim is characterized by that it comprises a Ti-added steel sheetcontaining 0.10% by weight or less of C, 1.0% or less of Mn, 0.15% orless of P, 0.1% or less of S, 0.1% or less of Al, 0.03 to 0.50% of Ti,and balance Fe accompanied by unavoidable impurities, having providedthereon a Ni—Mo alloy plating film, and which is then subjected to heattreatment.

[0011] The steel sheet for porcelain enameling according to the fourthclaim is characterized by that it comprises a Ti—Nb-added steel sheetcontaining 0.10% by weight or less of C, 1.0% or less of Mn, 0.15% orless of P, 0.1% or less of S, 0.1% or less of Al, 0.03 to 0.50% of Nb,and balance Fe accompanied by unavoidable impurities, having providedthereon a Ni—Mo alloy plating film, and which is then subjected to heattreatment.

[0012] The steel sheet for porcelain enameling according to the fifthclaims is characterized by that it comprises a Nb-added steel sheetcontaining 0.10% by weight or less of C, 1.0% or less of Mn, 0.15% orless of P, 0.1% or less of S, 0.1% or less of Al, 0.01 to 0.40% of Tiand 0.01 to 0.40% of Nb, provided that Ti and Nb in total not exceedingthe range of from 0.03 to 0.50%, and balance Fe accompanied byunavoidable impurities, having provided thereon a Ni—Mo alloy platingfilm, and which is then subjected to heat treatment.

[0013] The steel sheet for porcelain enameling according to the sixthclaims is characterized by that it comprises a B-added steel sheetcontaining 0.10% by weight or less of C, 1.0% or less of Mn, 0.15% orless of P, 0.1% or less of S, 0.1% or less of Al, 0.001 to 0.020% of B,and balance Fe accompanied by unavoidable impurities, having providedthereon a Ni—Mo alloy plating film, and which is then subjected to heattreatment.

[0014] The steel sheet for porcelain enameling according to the seventhclaims is characterized by that it is an steel sheet for porcelainenameling as described above, wherein the content of the elementspresent on the surface of the steel sheet within a depth of 100 nm fromthe surface as measured by ESCA (X-ray photoelectron spectroscopy) ischaracterized as such that the content of Ni is in a range of from 5 to50%, Mo is in a range of from 5 to 50%, and Fe is in a range of from 30to 90%, provided that the total of Ni, Mo, and Fe is 100%.

[0015] The method for producing a steel sheet for porcelain enamelingaccording to each of the claims 1 to 6 above of the present invention ischaracterized by that it comprises providing a Ni—Mo alloy plating oneach of the Al-killed steel sheet, high oxygen steel sheet, Ti-addedsteel sheet, Nb-added steel sheet, a Ti—Nb-added steel sheet, or B-addedsteel sheet, followed by applying a heat treatment thereto. As saidNi—Mo alloy plating, it is preferred that plating is provided in such amanner that the plated film contains Ni at a coverage of 1.5 to 20.0g/m² and Mo at a coverage of 0.4 to 7.0 g/m². Further, said heattreatment is performed in a temperature range of from 500 to 900° C.

[0016] Furthermore, the enamel product comprising an enamel layeraccording to the present invention is characterized by that it comprisesproviding an enamel layer on each of said steel sheet for porcelainenameling. Then, the method for producing an enamel product according tothe present invention comprises applying direct-on an overcoat enamel oneach of the steel sheets for porcelain enameling above, followed byapplying a heat treatment thereto.

[0017] The reason for confining each of the components in the Al-killedsteel sheet, high oxygen steel sheet, Ti-added steel sheet, Nb-addedsteel sheet, Ti—Nb-added steel, or B-added steel sheet above accordingto the present invention is as follows.

[0018] [C]

[0019] From the viewpoint of suppressing the generation of pores andblack specks on firing the enamel while assuring favorable workability,the content of C is set to 0.10% or less. To further improve theworkability in particular, the content of C is preferably set to 0.010%or less.

[0020] [Mn]

[0021] Manganese bonds with S to exhibit effects of suppressing crackingattributed to embrittlement from occurring during hot working and ofassuring the adhesion of enamel; hence, Mn should be incorporated at aconcentration of 1.0% or lower. If the content of Mn should exceed 1.0%,it hardens the material to result in an impaired ductility and pressmoldability.

[0022] [P]

[0023] Although P strengthens steel, its addition in excess degradesdeep drawability. Hence, the content of P is set to 0.15% or lower.

[0024] [S]

[0025] Sulfur accelerates surface roughening of the steel sheet onpickling as to improve the adhesion of the enamel by anchoring effect.However, S contained in excess is the cause of forming pores and blackspecks. Hence, the content of S is constrained to 0.1% or lower.

[0026] [Al]

[0027] Aluminum is added as a deoxidizer, but the addition in excessonly increases the cost. Hence, Al is added at an amount of 0.1% orless.

[0028] [O]

[0029] Oxygen is effective for improving the resistance against crackingand fish scales, however, the addition in excess increases blow holes ofthe continuously cast slab as to impair the product yield. Hence, incase of using high oxygen steel sheet, the addition of O is constrainedin a range of from 0.01 to 0.10%.

[0030] [Ti]

[0031] Titanium is effective for improving deep drawing properties andfor suppressing the generation of cracking and fish scale, and itsaddition of 0.03% or higher is necessary. However, the addition of Ti inexcess brings about the generation of surface defects and increase ofproduction cost. Thus, in case of using in a Ti-added steel sheet, theaddition of Ti is limited to a range of from 0.03 to 0.50%.

[0032] [Nb]

[0033] Similar to Ti, Nb has an effect on improving deep drawingproperties and on suppressing the generation of cracking and fishscales, and its addition of 0.03% or higher is necessary. However, theaddition of Nb in excess brings about the generation of surface defectsand increase of production cost. Thus, similar to Ti, in case of usingin a Nb-added steel sheet, the addition of Ti is limited to a range offrom 0.03 to 0.50%.

[0034] Furthermore, Ti may be used together with Nb, and in such a case,preferably, the addition of Ti is in a range of from 0.01 to 0.40% andthe addition of Nb is in a range of from 0.01 to 0.40%, provided that Tiand Nb in total is not lower than 0.03% but not exceeding 0.50%.

[0035] [B]

[0036] Boron is effective for improving resistance against cracking andfish scales, and its addition of 0.001% or higher is necessary. However,the excessive addition thereof results in an unstable slab properties;hence, in case of using in a B-added steel sheet, the addition of B isset in a range of from 0.001 to 0.020%.

BEST MODE FOR CARRYING OUT THE INVENTION

[0037] In the present invention, by applying Ni—Mo alloy plating on eachof the steel sheets of a low carbon Al-killed steel sheet, high-oxygensteel sheet, Ti-added steel sheet, Nb-added steel sheet, Ti—Nb-addedsteel sheet and B-added steel sheet, whose composition is controlled assuch described below, and by applying heat treatment thereto in order tocontrol the content of Ni, Mo, and Fe present in the surface of thesteel sheet for porcelain enameling in a predetermined range, it hasbeen found that excellent enamel adhesion properties are obtained evenon enameled products having the enamel provided by direct-on enameling.

[0038] In the present invention, the low-carbon Al-killed steel sheetcontains 0.10% by weight or less (wherein, % represents “% by weight”hereinafter) of C, 1.0% or less of Mn, 0.15% or less of P, 0.1% or lessof S, 0.1% or less of Al, and balance Fe accompanied by unavoidableimpurities. The high-oxygen steel sheet contains further added to thecomponents above, O at a content in a range of from 0.001 to 0.10%; theTi-added steel sheet contains further added to the components above, Tiin a range of from 0.03 to 0.50%; the Nb-added steel sheet containsfurther added to the components above, Nb in a range of from 0.03 to0.50%; the Ti—Nb-added steel sheet contains further added to thecomponents above, Ti in a range of from 0.01 to 0.40% and Nb in a rangeof from 0.01 to 0.40%, provided that Ti and Nb in total do not exceed arange of from 0.03 to 0.50%; and the B-added steel sheet containsfurther added to the components above, B in a range of from 0.001 to0.020%.

[0039] A slab is then produced from the steel having its compositionadjusted to the range above by means of continuous casting. Theresulting slab is then hot rolled, or, hot rolled after re-heating.Then, after descaling by a known method such as sulfuric acid picklingand the like, the resulting product is cold rolled at a reduction ofabout 50 to 95%, annealed at a temperature of recrystallizationtemperature or higher but lower than the Ac₃ point by means of core boxannealing or by continuous annealing process, and subjected to refiningrolling at a reduction of about 0.1 to 5% to obtain the steel sheet foruse in the present invention.

[0040] Subsequently, a Ni—Mo alloy plating is provided to the resultingsteel sheet. The alloy plating may be performed by either means ofelectroless plating or electrolytic plating, but from the ease ofcontrol, preferred is electrolytic plating. As the plating bath, thereis used an aqueous solution having supplied thereto Ni ions in the formof a salt of an inorganic acid, such as a sulfate, a nitrate, a halide,etc. and Mo ions in the form of an ammonium salt of a metallic acid saltand the like, having further added thereto, as a complexing agent, anorganic acid such as citric acid, tartaric acid, or a malic acid or asalt thereof. Then, after adding an acid or an alkali to the resultingaqueous solution to control the pH in a range of from 2 to 4, andadjusting the bath temperature in a range of from 30 to 50° C., directcurrent electrolysis is performed at a current density of from 5 to 30A/dm² by using a Ni plate as the anode. Preferably, degreasing treatmentand pickling treatment are performed on the steel sheet by an ordinarymethod just before applying the plating above, because the steel sheetbefore plating tends to be oxidized with passage of time or be broughtinto contact with oils and fats.

[0041] Concerning the components in the coating formed by alloy platingabove, the coating contains 1.5 to 20.0 g/m², preferably 2.0 to 6.0 g/m²of Ni, and 0.4 to 7.0 g/m², preferably 1.0 to 2.0 g/m² of Mo. Thecontent can be obtained by fluorescent X-ray spectroscopy. In case thecontent of Ni and Mo in the plating should fall outside the range above,favorable adhesion cannot be assured between the enamel and the steelsheet, because the elements Fe, Ni, and Mo in the surface of the steelsheet cannot be pertained in the preferred range on applying heattreatment after plating as described below.

[0042] Then,, heat treatment is performed after providing the Ni—Moalloy plating on the steel sheet as described above. The heat treatmentis carried out in a manner similar to annealing generally performed onan ordinary steel sheet. As the heat treatment, i.e., as the annealing,there can be used either core box annealing or continuous annealingwithout any problem. Although depending on the content of Fe, Ni, and Mopertained on the surface of the steel sheet after heat treatment, theconditions of annealing are, heating in the temperature range of from500 to 900° C. for a time duration of from 1 minute to 15 hours under areducing atmosphere of a gaseous decomposed ammonia or gaseous HNX, morepreferably, heating in the temperature range of from 550 to 750° C. fora time duration of from 1 minute to 8 hours is performed.

[0043] In the manner described above, steel sheet for porcelainenameling according to the present invention can be obtained. By theheat treatment described above, Ni and Mo diffuse into the steel sheet,and Ni and Mo undergo mutual diffusion as to change the content of Fe,Ni, and Mo present in the surface of the steel sheet. The content of theelements present in the surface of the steel sheet can be measured byusing a surface analyzer of, for instance, ESCA. After the heattreatment, the content of Fe, Ni, and Mo present in the surface of thesteel sheet to a depth of 100 nm from the surface is, 5 to 50% Ni, 5 to50% Mo, and 30 to 90% Fe; preferably, 5 to 30% Ni, 5 to 30% Mo, and 40to 70% Fe; further, by adjusting the content of Ni, Mo, and Fe in such amanner that the total thereof should become 100%, an excellent adhesionproperties of enamel with the steel sheet can be obtained. The “depth”as referred herein is expressed by the depth obtained on the standardsample SiO₂ by etching for the same time duration as that employed inAr-ion etching the surface of the steel sheet after heat treatment. Bythus performing heat treatment after applying Ni—Mo alloy plating, aMo—Ni layer containing Fe at a certain concentration or higher is formedon the surface of the plated steel sheet, and thereby a favorable enameladhesion can be assured. In case the content of Fe, Ni, and Mo shouldfall outside the range above, a favorable enamel adhesion becomesunfeasible.

[0044] The steel sheet for porcelain enameling thus obtained asdescribed above can be enameled as it is in the state of flat platewithout processing, or may be enameled after shaping and processing itinto the desired shape by applying bending and drawing. Furthermore, byusing the steel sheet for porcelain enameling according to the presentinvention, a porcelain enameled product with excellent enamel adhesioncan be obtained while omitting pickling and nickel treatment processsteps. In particular, in case of steel sheet for porcelain enameling foruse in a porcelain enameled product in which porcelain enamel is appliedby direct-on process, it is necessary to etch iron for an amount of 30to 40 g/m² in the pickling step of the pretreatment, and to depositnickel at a coverage of 0.6 to 1.5 g/m² on the surface thereof. All ofthese process steps can be excluded in the case of using the steel sheetfor porcelain enameling according to the present invention. Furthermore,the porcelain enameled products can be obtained by direct-on enamelfinishing of the cover coat porcelain enamel, ground coat enamelfinishing, ground coat and cover coat enamel finishing, decoratingenamel finishing, etc., and the steel sheet for porcelain enamelingaccording to the present invention is applicable to all of theseporcelain enameled products.

EXAMPLES

[0045] The present invention is described in further detail by way ofexamples below.

[0046] [Production of steel sheet for porcelain enameling]

[0047] Steels (A1 to F3) having the composition as shown in Table 1 wereeach molten, and 22 types of slabs were produced. The slabs were eachheated to 1160° C., and were each hot rolled at a finish temperature of880° C. to obtain each of the hot rolled sheets 2.8 mm in thickness,which were each taken up into a coil at 650° C. Then, while recoiling,descaling and sulfuric acid pickling were applied to the sheet, andafter applying cold rolling thereto to obtain a steel sheet 0. 8 mm inthickness, continuous annealing at 830° C. was applied for 75 seconds,and refining rolling at a reduction of 0.5% was performed thereon toobtain a cold rolled steel sheet.

[0048] After performing alkali degreasing to the cold rolled steel sheetby an ordinary means and by applying pickling thereto by using anaqueous sulfuric acid solution, electric plating of a Ni—Mo alloy wasapplied under the conditions below by using the following plating bathto obtain coverage as shown in Table 2. Then, by applying heat treatmentunder the condition shown in Table 2 in gaseous decomposed ammonia, eachof the steel sheets for porcelain enameling shown as Examples 1 to 23 inTable 2 was produced. The content of the elements Fe, Ni, and Mo presentin the surface of the steel sheet for porcelain enameling was measuredby using ESCA. <Plating bath> Nickel sulfate: 82 g/L Ammonium molybdate:48 g/L Sodium citrate: 88 g/L <Plating condition> pH: 3.0 Bathtemperature: 40° C. Current density: 20 A/dm² Anode: Nickel plate

[0049] TABLE 1 Chemical composition of steel for porcelain enameling Feand No. of Chemical composition of steel (% by weight) unavoidable SteelC Mn P S Al O Ti Nb B impurities A1 0.0011 0.16 0.008 0.004 0.064 — — —— balance A2 0.0049 0.25 0.017 0.005 0.002 — — — — balance A3 0.01000.36 0.011 0.030 0.070 — — — — balance A4 0.0072 1.00 0.013 0.015 0.030— — — — balance A5 0.0067 0.29 0.150 0.015 0.030 — — — — balance A60.0046 1.00 0.019 0.100 0.030 — — — — balance A7 0.0023 0.29 0.013 0.0220.100 — — — — balance B1 0.0031 0.30 0.013 0.008 0.010 0.001 — — —balance B2 0.0027 0.28 0.019 0.014 0.005 0.034 — — — balance B3 0.00260.33 0.012 0.012 0.008 0.100 — — — balance C1 0.0029 0.19 0.018 0.0220.038 — 0.030 — — balance C2 0.0024 0.12 0.025 0.033 0.030 — 0.079 — —balance C3 0.0031 0.23 0.022 0.030 0.027 — 0.500 — — balance D1 0.00450.20 0.022 0.032 0.038 — — 0.030 — balance D2 0.0029 0.14 0.025 0.0260.051 — — 0.068 — balance D3 0.0032 0.19 0.019 0.029 0.023 — — 0.500 —balance E1 0.0052 0.10 0.029 0.039 0.042 — 0.010 0.240 — balance E20.0037 0.18 0.018 0.024 0.036 — 0.216 0.193 — balance E3 0.0044 0.120.020 0.033 0.043 — 0.332 0.010 — balance F1 0.0062 0.24 0.019 0.0080.084 — — — 0.001 balance F2 0.0038 0.29 0.011 0.017 0.099 — — — 0.0097balance F3 0.0042 0.19 0.016 0.011 0.054 — — — 0.020 balance

[0050] TABLE 2 Steel sheets for porcelain enameling Content of elementsCoverage of Heat treatment on steel Examples and plating conditionssurface Comparative No. of Ni Mo Temperature (% by weight) ExamplesSteel (g/m²) (g/m²) (° C.) Time Fe Ni Mo Example 1 A1 1.5 0.6 900 1minute 55.8 35.1 9.1 Example 2 A2 4.5 1.1 630 8 hours 37.4 44.5 18.1Example 3 A3 6.0 2.0 750 3 hours 77.0 7.7 15.3 Example 4 A4 20.0 4.8 7005 hours 43.1 40.2 16.7 Example 5 A5 1.5 6.8 750 3 hours 45.1 5.1 49.8Example 6 A6 4.6 1.2 550 8 hours 34.3 45.1 20.6 Example 7 A7 1.5 0.4 7501 minute 88.9 5.6 5.5 Example 8 B1 3.3 1.2 750 10 hours 88.8 5.6 5.6Example 9 B2 2.5 1.2 600 10 hours 32.8 19.9 47.3 Example 10 B3 3.5 1.3700 5 hours 31.4 33.8 24.8 Example 11 C1 4.8 1.4 750 1 hour 55.5 25.519.0 Example 12 C2 5.7 1.6 600 8 hours 33.4 40.5 26.1 Example 13 C3 4.31.2 620 8 hours 33.6 47.2 19.2 Example 14 D1 2.0 0.4 900 1 minute 53.837.3 7.9 Example 15 D2 6.0 1.8 500 15 hours 30.5 48.8 20.7 Example 16 D35.4 3.1 750 2 hours 64.1 10.4 25.5 Example 17 E1 15.6 7.0 700 5 hours47.1 28.2 24.7 Example 18 E2 2.5 0.8 650 5 hours 38.4 47.3 14.3 Example19 E3 4.4 1.2 650 6 hours 35.6 47.6 16.8 Example 20 F1 4.6 1.3 640 7hours 35.2 47.0 17.8 Example 21 F2 5.9 1.8 740 3 hours 72.1 11.5 16.4Example 22 F3 4.2 1.5 620 9 hours 36.6 48.1 15.3 Example 23 A2 3.7 1.4650 5 hours 37.5 35.5 27.0 Comp. Ex. 1 E2 3.1 — 550 5 hours 1 85.8 —Comp. Ex. 2 D3 22.5 8.8 400 1 hour 3.1 65.7 31.2 Comp. Ex. 3 B2 1.3 —950 1 hour 95.3 4.7 —

[0051] Furthermore, as Comparative Example, steel type No. D3 waselectrically plated by using the plating bath above under the platingconditions above in such a manner to result in the coverage shown inComparative Example 2 shown in Table 2. For steel type Nos. E2 and B2,Ni was electrically plated under the conditions shown below to result inthe coverage shown in Comparative Examples 1 and 3 shown in Table 2, andthe resulting products were subjected to heat treatment under conditionsshown in Table 2 in gaseous decomposed ammonia. Thus were obtained steelsheets for porcelain enameling given as Comparative Examples 1 to 3 intable 2. The content of elements Fe, Ni, and Mo that are present on thesurface of the steel sheets for porcelain enameling was measured byusing ESCA. <Plating bath> Nickel sulfate: 300 g/L Nickel chloride:  45g/L Boric acid:  30 g/L <Plating condition> pH: 4.0 Bath temperature:55° C. Current density: 10 A/dm² Anode: Nickel plate

[0052] [Evaluation of Properties]

[0053] The workability, adhesion of the porcelain enamel provided bydirect-on enameling of the cover coat enamel, and the appearance wereevaluated on each of the steel sheets for porcelain enameling thusobtained.

[0054] <Workability>

[0055] The workability of the steel sheets for porcelain enameling wasevaluated by working them each into Yamada-type cup (draw ratio: 2.2),and the moldabilty into the cup was evaluated; further, peeling test wasperformed on the worked portion by using a cellophane tape. Theworkability was evaluated under evaluation standards as follows. Theresults are shown in Table 3.

[0056] Good: Good cup moldability and peeling test result were obtained

[0057] Fair: Good cup moldability was obtained, and peeling off occurredon 5% or less of the tested portion

[0058] Poor: Good cup moldability was obtained, and peeling off occurredon 5% or more of the tested portion

[0059] Bad: Cup moldability and peeling test result were both poor

[0060] After providing cover coat porcelain enamel (No. 02-1103/100,produced by FERRO ENAMELS (JAPAN) LIMITED) to the steel sheets forporcelain enameling above by direct-on enameling in such a manner thatthe thickness on firing should become about 120 μm and drying, theresulting product was fired at 800° C. for 3 minutes under theatmosphere in a baking furnace to obtain the samples. The samples werethen evaluated in the manner described below on the adhesion of theporcelain enamel and on the appearance.

[0061] <Adhesion>

[0062] The adhesion of the porcelain enamel was evaluated by PEI methodin the following manner.

[0063] A steel ball 25 mm in diameter was pressed against the flat sheetportion of the sample and the sample was deformed by applying a force of8.9 kN using a hydraulic press, and 169 metallic conductive probes werepressed to the deformed portion to apply electric current. Theinsulation was evaluated in accordance with the following equation.

Insulation(%)=(n/169)×100,

[0064] where, n represents the number of probes showing no conduction.From the value of insulation (%) thus obtained by the equation above,the adhesion of the enamel was evaluated in accordance with thefollowing evaluation standard.

[0065] Good: Insulation >85%

[0066] Fair: 80% <Insulation <85%

[0067] Poor: Insulation <80%

[0068] The results are given in Table 3.

[0069] <Appearance>

[0070] The appearance of the enamel was evaluated by cutting out tentest pieces each 30 cm×30 cm in size from a single sample. The testpieces were each visually observed for the generation of pores, blackspecks, and cracking and fish scales, and the number of the generateddefects was counted. Evaluation was made in accordance with the standardas follows.

[0071] -Pores and Black Specks-

[0072] Good: No pores and black specks were observed.

[0073] Fair: Less than ten pores or black specks in total were observedon ten test pieces.

[0074] Poor: Ten or more pores or black specks in total were observed onten test pieces.

[0075] -Cracking and Fish Scale-

[0076] Good: No cracking and fish scales were observed.

[0077] Fair: Less than ten cracking and fish scales in total wereobserved on ten test pieces.

[0078] Poor: Ten or more cracking and fish scales in total were observedon ten test pieces. TABLE 3 Results of Evaluated Properties Result ofEvaluated Properties Appearance Pores Examples and and CrackingComparative Process- Adhesion of black and fish Examples Ability enamelspecks scales Example 1 Good Good Good Good Example 2 Good Good GoodGood Example 3 Good Good Good Good Example 4 Good Good Good Good Example5 Good Good Good Good Example 6 Good Good Good Good Example 7 Good GoodGood Good Example 8 Good Good Good Good Example 9 Good Good Good GoodExample 10 Good Good Good Good Example 11 Good Good Good Good Example 12Good Good Good Good Example 13 Good Good Good Good Example 14 Good GoodGood Good Example 15 Good Good Good Good Example 16 Good Good Good GoodExample 17 Good Good Good Good Example 18 Good Good Good Good Example 19Good Good Good Good Example 20 Good Good Good Good Example 21 Good GoodGood Good Example 22 Good Good Good Good Comp. Ex. 1 Good Good Poor GoodComp. Ex. 2 Bad Good Poor Poor Comp. Ex. 3 Good Poor Good Poor Comp. Ex.4 Poor Poor Poor Poor

[0079] The results are given in Table 3.

[0080] As shown in Table 3, the steel sheet for porcelain enamelingaccording to the present invention has excellent workability, and amongthem, Ti-added steel and B-added steel showed particularly superiorproperties. Furthermore, enameled products produced by direct-onenameling of cover coat porcelain enamel having excellent enameladhesion and appearance were obtained for the entire scope of thepresent invention.

[0081] In contrast to above, in Comparative Example 1, good enameladhesion was obtained, but the cup workability was poor; in ComparativeExample 2, on the contrary, the cup workability was good, but the enameladhesion was poor; and in Comparative Example 3, poor results wereobtained on both properties. Concerning enamel appearance, pores andblack specks as well as cracking and fish scales were observed onComparative Examples 1 and 3, and in Comparative Example 2, cracking andfish scales were observed. Hence, the enamel appearance was poor for allof the comparative samples.

[0082] [Production of Porcelain Enamel Products]

[0083] Porcelain Enamel products were produced in the following mannerby using the steel sheet for porcelain enameling according to thepresent invention.

[0084] The steel sheets for porcelain enameling given as Examples 21 and23 in Table 2 were each pressed into a shape of a saucepan having aninner diameter of 160 mm and a depth of 110 mm, and into a shape of thetop plate of an oil stove 220 mm in length, 400 mm in width, and 8 mm indepth. To the base metal for enameling thus obtained, porcelain enamelwas applied in 4 methods, and the resulting products were fired toobtain the porcelain enamel products.

[0085] <Porcelain Enamel>

[0086] Ground coat enamel: 03-1226, produced by FERRO ENAMELS (JAPAN)LIMITED

[0087] Cover coat enamel: 02-2105, produced by FERRO ENAMELS (JAPAN)LIMITED

[0088] <Enameling>

[0089] (1) Ground Coat Enamel Finish (Applying Ground Coat EnamelOnce—Firing Once)

[0090] To the saucepan and the oil stove top plate obtained by pressworking above as the base metal for enameling, the ground coat enamel03-1226 was applied to obtain a fired coating about 100 μm in thickness.The resulting product was dried and fired in a firing furnace at 820° C.for 5 minutes.

[0091] (2) Cover Coat Enamel Finishing on Ground Coat Enamel-FinishedSurface (Applying Enamel Twice—Firing Twice)

[0092] After applying a ground coat enamel in the same manner as aboveand firing, the cover coat enamel above, 02-2105, was applied to thesurface in such a manner that the fired thickness of about 100 μm wouldresult. The resulting product was dried and fired in a firing furnace at820° C. for 5 minutes.

[0093] (3) Ground Coat Enamel+Cover Coat Enamel Finishing (ApplyingEnamel Twice—Firing Once)

[0094] To the same saucepan and the oil stove top plate as aboveobtained as the base metal for enameling, the ground coat enamel 03-1226described above was applied to obtain a fired coating about 80 μm inthickness. Then, without firing, the cover coat enamel described above,02-2105, was applied in such a manner to obtain a fired coating about120 μm in thickness. The resulting product was dried and baked in afiring furnace at 820° C. for 5 minutes.

[0095] (4) Cover Coat Enamel Finishing (Direct-On Applying Cover CoatEnamel—Firing Once)

[0096] To the same saucepan and the oil stove top plate as aboveobtained as the base metal for enameling, the cover coat enameldescribed above, 02-2105, was applied in such a manner to obtain a firedcoating about 120 μm in thickness. The resulting product was dried andfired in a firing furnace at 820° C. for 5 minutes.

[0097] The saucepans and the oil stove top plates obtained as theporcelain enamel products according to (1) to (4) above were subjectedto evaluation for adhesion and appearance.

[0098] <Adhesion>

[0099] Similar to the evaluation performed on the steel sheet forporcelain enameling described hereinbefore, PEI method was used for theevaluation.

[0100] <Appearance>

[0101] The appearance of the enamel products was visually observed toevaluate the generation of pores, black specks, pinholes, cracking andfish scales, etc. The results are given in Table 4.

[0102] As shown in Table 4, similar to the case of applying firing aground coat enamel and applying a cover coat enamel, or to the case oftwice enameling, i.e., to the case of applying a ground coat enamel andthen applying a cover coat enamel and firing, the porcelain enamelproducts according to the present invention, which are obtained bydirect-on enameling of a ground coat enamel or a cover coat enamel,exhibit excellent enamel adhesion and appearance. TABLE 4 Porcelainenamel Enameling method product Appearance Adhesion Ground enamelSaucepan Good; Good finishing Top plate of free from (Applying ground)oil stove pores, coat enamel once- black firing once) specks, Cover coatenamel Saucepan pinholes, finishing on Top plate of cracking and groundcoat oil stove fish scales, enamel-finished etc. surface (Applying)enamel twice- firing twice) Ground coat Saucepan enamel + Cover Topplate of coat enamel oil stove finishing (Applying enamel twice − firingonce) Cover coat enamel Saucepan finishing Top plate of (Direct-on oilstove applying cover coat enamel − firing once)

[0103] Furthermore, the steel sheet for porcelain enameling according tothe present invention is applicable, not only as the base metal ofenamel products, but also as base metal for forming thereon inorganic ororganic coating films.

[0104] Industrial Applicability p As described above, the presentinvention enables a steel sheet for porcelain enameling having excellentworkability and enamel adhesion by applying a Ni—Mo alloy plating on lowcarbon Al-killed steel sheet, high-oxygen steel sheet, Ti-added steelsheet, Nb-added steel sheet, Ti—Nb-added steel sheet or B-added steelsheet, and then performing heat treatment thereto. Furthermore, bycontrolling the amount of Ni, Mo, and Fe present on the surface of thesteel sheet in a predetermined range, and by then providing a cover coatenamel by direct-on enameling and firing, porcelain enamel productshaving excellent enamel adhesion and appearance well comparable to theenamel products resulting from twice applying the cover coat and groundcoat enamel can be obtained.

1. A steel sheet for porcelain enameling comprising an Al-killed steelsheet containing 0.10% by weight or less (wherein, % represents “% byweight” hereinafter) of C, 1.0% or less of Mn, 0.15% or less of P, 0.1%or less of S, 0.1% or less of Al, and balance Fe accompanied byunavoidable impurities, having provided thereon a Ni—Mo alloy platingfilm, and which is then subjected to heat treatment.
 2. A steel sheetfor porcelain enameling comprising a high oxygen steel sheet containing0.10% by weight or less of C, 1.0% or less of Mn, 0.15% or less of P,0.1% or less of S, 0.1% or less of Al, 0.001 to 0.10% of 0, and balanceFe accompanied by unavoidable impurities, having provided thereon aNi—Mo alloy plating film, and which is then subjected to heat treatment.3. A steel sheet for porcelain enameling comprising a Ti-added steelsheet containing 0.10% by weight or less of C, 1.0% or less of Mn, 0.15%or less of P, 0.1% or less of S, 0.1% or less of Al, 0.03 to 0.50% ofTi, and balance Fe accompanied by unavoidable impurities, havingprovided thereon a Ni—Mo alloy plating film, and which is then subjectedto heat treatment.
 4. A steel sheet for porcelain enameling comprising aNb-added steel sheet containing 0.10% by weight or less of C, 1.0% orless of Mn, 0.15% or less of P, 0.1% or less of S, 0.1% or less of Al,0.03 to 0.50% of Nb, and balance Fe accompanied by unavoidableimpurities, having provided thereon a Ni—Mo alloy plating film, andwhich is then subjected to heat treatment.
 5. A steel sheet forporcelain enameling comprising a Ti—Nb-added steel sheet containing0.10% by weight or less of C, 1.0% or less of Mn, 0.15% or less of P,0.1% or less of S, 0.1% or less of Al, 0.01 to 0.40% of Ti and 0.01 to0.40% of Nb, provided that Ti and Nb in total not exceeding the range offrom 0.03 to 0.50%, and balance Fe accompanied by unavoidableimpurities, having provided thereon a Ni—Mo alloy plating film, andwhich is then subjected to heat treatment.
 6. A steel sheet forporcelain enameling comprising a B-added steel sheet containing 0.10% byweight or less of C, 1.0% or less of Mn, 0.15% or less of P, 0.1% orless of S, 0.1% or less of Al, 0.001 to 0.020% of B, and balance Feaccompanied by unavoidable impurities, having provided thereon a Ni—Moalloy plating film, and which is then subjected to heat treatment.
 7. Asteel sheet for porcelain enameling as claimed in one of claims 1 to 6,wherein the content of the elements present on the surface of the steelsheet within a depth of 100 nm from the surface is characterized as suchthat the content of Ni is in a range of from 5 to 50%, Mo is in a rangeof from 5 to 50%, and Fe is in a range of from 30 to 90%, provided thatthe total of Ni, Mo, and Fe is 100%.
 8. A method for producing a steelsheet for porcelain enameling, which is characterized by that itcomprises providing a Ni—Mo alloy plating on the Al-killed steel sheet,high oxygen steel sheet, Ti-added steel sheet, Nb-added steel sheet,Ti—Nb-added steel sheet, or B-added steel sheet claimed in one of claims1 to 6, followed by applying a heat treatment thereto.
 9. A method forproducing a steel sheet for porcelain enameling as claimed in claim 8,wherein said Ni—Mo alloy plating is characterized by that it is platedas such that the plating film contains Ni at a coverage of 1.5 to 20.0g/m² and Mo at a coverage of 0.4 to 7.0 g/m².
 10. A method for producinga steel sheet for porcelain enameling as claimed in claim 8 or 9,wherein said heat treatment is performed in a temperature range of from500 to 900° C.
 11. A porcelain enameled product comprising an enamellayer provided on the steel sheet for porcelain enameling as claimed inone of claims 1 to
 7. 12. A method for producing a porcelain enameledproduct, characterized by that the method comprises once applying acover coat enamel on the steel sheet for porcelain enameling as claimedin one of claims 1 to 7, followed by applying a heat treatment thereto.